Amplitude behaviour of geodesic acoustic modes in the ASDEX Upgrade tokamak

The amplitude behaviour of the geodesic acoustic mode (GAM), a low frequency (5-25 kHz) coherent plasma turbulence-generated E-r x B zonal flow oscillation, is studied in the ASDEX Upgrade tokamak using Doppler reflectometry. Across the tokamak edge the GAM displays one or more maxima a few cm wide, coinciding with radial plateaus in the GAM frequency, suggesting nested zonal flow layers. The GAM peak amplitude ranges between 0.2 and 0.8 km s(-1), corresponding to several tens of per cent of the mean E x B flow velocity. In limiter configurations, the GAM amplitude is found to decrease inversely with the plasma vertical elongation 1.1 < kappa < 1.4, but rises again in X-point divertor configurations at higher kappa similar to 1.6. At low kappa the GAM amplitude increases with the safety factor q, consistent with damping effects, but becomes less sensitive to q at high.. A positive linear dependence on the electron temperature gradient, an indicator of the linear turbulence drive, is also observed.